Hot section components commonly found in the combustor and turbine section of modern gas turbine engines are made of high temperature alloys selected from the group consisting of nickel-based superalloys, iron based superalloys, cobalt-based superalloys and combinations thereof. These superalloys have been developed to meet the demands of higher operating temperature while being able to survive the severe environment of the hot section of the gas turbine. In order to improve the survivability of the component at high temperatures under corrosive and oxidative conditions, protective coating systems typically are applied to the components. These coating systems typically include an environmental coating, which also serves as a bond coat, and usually a thermal barrier coating overlying the environmental or bond coat.
The environmental coatings, which may be used without thermal barrier coating, or bond coatings, are typically metallic overlay coatings of MCrAl(X) where M is an element selected from the group consisting of Co and Ni and combinations thereof and (X) is an element selected from the group of rare earth elements and Y, Hf, W, Zr, La and combinations thereof, or diffusion aluminide coatings such as NiAl or modified NiAl that includes an element such as Pt, Rh or Pd. The optional thermal barrier coatings are ceramic materials, usually a yttria-modified zirconia. The thermal barrier coatings are applied over environmental coating or bond coatings, as the adhesion of the thermal barrier coatings to these coatings is far superior to the long-term adhesion of these coatings to the base metal superalloys.
Various well-known methods are used to apply both the environmental coatings and the ceramic thermal barrier coatings to the hot section components. As the operating temperatures have become higher and the environmental conditions have become harsher, additional cooling in the form of serpentine internal cooling passageways has been added, where practical, to these hot section components. Of course, because the surfaces of these passageways are also subject to environmental attack, it has been necessary to apply coatings to provide environmental protection to these surfaces. While a wide variety of methods are available to apply environmental coatings to the external surfaces of the hot section components, not all of these methods can be used successfully for coating of the serpentine internal passageways. For example, line-of-sight methods such as for example, EB-PVD, sputtering, directed vapor deposition (DVD), cathodic arc and thermal spray, are ineffective, and plating methods are not easily employed.
In order to successfully provide an environmental protective coating to the serpentine internal cooling passages of a hot section component such as a turbine airfoil, the current state of the art utilizes a gas phase of the coating species, aluminum or chromium and other reactive elements (Hf, Y, Zr etc.) and combinations thereof, which is passed through the serpentine passages and deposited on the internal surfaces. These gas phases are generated at high temperatures by well-known processes such as chemical vapor deposition (CVD), vapor phase aluminiding (VPA) and above-the-pack deposition. Current commercial CVD and VPA techniques, in efforts to be cost effective, are usually used to simultaneously coat both the surfaces of the internal passageways and the external surfaces of the component. The rate of coating deposition on the external surfaces can be greater than the deposition rate on the internal surfaces. However, the thin airfoil walls are an important consideration, as the metallic vapor combines with the substrate surface material to form the coating. This decreases the effective substrate wall thickness as substrate elements combine with the metallic vapor to form a coating and undesirably leads to wall consumption over time. Dimensional accuracy of the component is also important, since the component must fit up with mating surfaces, for example, platform edges of a turbine blade, so that even if a coating is permitted, its thickness must be carefully controlled. In some circumstances, excessive coating can adversely affect the base metal composition by altering the composition and perhaps generating undesirable phases. The alteration of the base material can cause a decrease in the load-bearing capability of the base material. This can result in a shortened life of the component, and can adversely affect the ability to repair and reuse the component. And certainly, if the coating is built up to an excessive thickness, it no longer performs as a coating, but rather behaves as bulk material, which also is undesirable, as these bulk materials generally lack the required mechanical properties.
In order to protect these surfaces from any coating, or from a coating build up that is excessive, it is necessary to mask these surfaces while the internal serpentine passageways are being coated. These masking methods are well known and described in, for example, U.S. Pat. Nos. 4,464,430, 3,801,357 to Baldi, U.S. Pat. No. 4,128,522 to Elam. U.S. Pat. No. 3,647,497 to Levine et al., and U.S. Pat. No. 4,978,558 to Lamm et al. Each of these can be effective, but typically require application of a plurality of masking layers with intermediate operations. This is both time-consuming and expensive. These masking materials often make use of sacrificial layers of metal such as Ni and Cr and combinations thereof. These typically are applied as tapes, slurries and putties that perform as “getters” of the aluminiding gases, whose purpose is to cause a reaction with the sacrificial layer before the gases can reach the surface of the component intended to be protected. In addition, certain masking methods utilize materials that either burn off leaving small portions of the substrate exposed or form cracks that permit migration and deposition of the gaseous phase onto the substrate. Additionally, the masking layers can be tightly adherent and difficult to remove.
What is needed is a masking material that can be readily applied over surfaces that desirably are not to be exposed, but which otherwise would be exposed to the aluminum-bearing gaseous phase during coating operations at high temperatures. The masking material must be easy to apply, capable of surviving high temperatures and must form a continuous, crack-free protective layer over the surfaces that are not being coated. The masking material must not react with the substrate material. Furthermore, while the masking material must bond to the surface, it must be readily and completely removable from the surface without the need to heavily work the surface mechanically or subject the surface to harsh chemicals.